the neuroretinal rim correspond to regional visual field defects in eyes with normal-tension glaucoma [23].

20.4.3  Canon Laser Blood

Flowmetry

Feke et al. found significant differences in blood flow in response to postural changes between primary OAG patients and controls, suggesting underlying autoregulatory dysfunction in glaucoma patients [24].

20.4.4  Laser Doppler Flowmetry

Boehm et al. found decreased blood flow in the temporal neuroretinal rim compared to nasal blood flow in healthy subjects, suggesting potential vulnerability of the temporal neuroretinal rim to ischemic insult [25].

venous filling times, prolonged AVP time, and reduced velocity in the retinal circulation compared to controls [28, 29]. The AVP time correlated significantly with the optic nerve head size, visual field global indices, mean deviation, pattern standard deviation and corrected pattern standard deviation, and contrast sensitivity in normal tension glaucoma patients [30].

20.4.7  ICG-Scanning Laser

Ophthalmoscopy

Marengo et al. found that subjects with advanced glaucoma show prominent increase in the cup/disc area ratio, as well as marked capillary dropout [31]. O’Brart et al. demonstrated areas of hypofluorescence in the peripapillary region in late-phase angiograms in 68% of glaucomatous eyes compared with 20% of control eyes [32].

20.4.5  Retinal Vessel Analyzer

Nagel et al. demonstrated that retinal vein diameter autoregulatory response to acute IOP elevation is diminished in patients with primary OAG [26]. Garhofer et al. found the autoregulatory response to flicker-induced vasodilation of retinal veins is significantly diminished in patients with glaucoma compared with healthy volunteers [27].

20.4.6  FA-Scanning Laser

Ophthalmoscopy

Harris at al. showed that OAG patients had fluorescein filling defects in the superficial part of the optic disc and choroid, delayed arm-to-retina and retinal arterial and

20.4.8  Pulsatile Ocular Blood Flowmeter/

Pascal DCT

James et al. showed reduced OPA and pulsatile ocular blood flow in high and normal-pressure glaucoma [33]. von Schulthess et al. suggested that an early drop of more than 2.0 mmHg in OPA after trabeculectomy may be a good prognostic parameter for successful long-term control of IOP [34] (Fig. 20.3).

Summary for the Clinician

››Patients with normal and high tension OAG have been found to have vascular abnormalities in the retinal, choroidal, and retrobulbar circulations.

››Autoregulatory dysfunction has been reported in OAG patients.

Fig. 20.3  Pulsatile ocular blood flowmeter (POBF). The left picture: The POBF device. This pneumotometer measures the intraocular pressure (IOP) in real time. It is based on the principle that blood volume in the eye increases with systolic pulses and decreases during diastole. The transient change in IOP is

used to calculate change in ocular volume. The right picture: The probe is slit-lamp mounted and operated similarly to Goldmann’s applanation tonometer. It is gently placed on the cornea until three consecutive measurements are recorded.

20.5  How Are the Results of Blood Flow Measuring Devices Interpreted and Are There Limitations to These Blood Flow Imaging Techniques?

Since all the above techniques provide us with quantitative hemodynamic data but not with the absolute flow, it is necessary to evaluate their results with caution. Currently, most of these techniques are not used in the clinic and are primarily used for research purposes.

20.5.1  Color Doppler Imaging

Parallel changes in CDI peak systolic and end diastolic velocities may be interpreted as changes in volumetric blood flow in the same direction [35]. Conversely, an increase in the peak systolic velocity may be interpreted as only proximal arterial stenosis. An increase in resistive index may reflect arterial stenosis distal to the point of measurement. Increased resistive index in the central retinal and short posterior ciliary arteries,

without an increase in the ophthalmic artery, may simply be due to elevated IOP [36].

20.5.2  Heidelberg Retinal Flowmeter

The Heidelberg retinal flowmeter provides blood flow data in subcapillary resolution. The flow measurements are in arbitrary units and their exact correlation to real flow data remains unclear. The software within the device may cause distortion since it includes areas with no vessels. A pixel-by-pixel analysis has been developed to exclude areas of no perfusion and has been found to be highly reproducible [37].

20.5.3  Canon Laser Blood Flowmetry

Retinal volumetric blood flow is calculated in absolute units. Although several studies report reproducible measurements of retinal blood flow in normal subjects,